The US has issued a strong warning against Nigeria's proposed amendment of its constitution to allow President Olusegun Obasanjo to seek a third term.
Mr Obasanjo's second term in office ends next year
A US embassy statement said limits on terms in office should be respected to help reinforce the democratic process.
President Obasanjo has so far not said whether he will seek a third term but his supporters say he should run again.
Nigerians are split over the issue, which is being debated by the National Assembly over the next few weeks.
"The United States respects the right of any country to amend its constitution through democratic, transparent and legal means," the statement from the US embassy said.
But it warned that many Nigerians feel the government has not run the constitutional review process fairly or transparently.
Observers say the statement is the strongest yet from the US government.
Supporters of President Obasanjo say he should stand for another term.
Vice-President Atiku Abubakar would like to stand for president
However, the government itself is divided over the issue.
The vice-president, who also wants the top job, has declared himself against the amendment.
Atiku Abubakar has helped Mr Obasanjo win two elections and has held the position of vice-president since 1999.
Opponents of the constitutional change argue that the presidency needs to rotate among people from different regions and ethnic groups.
For the amendment to pass, it requires a two-thirds majority in both houses of the assembly and it needs the backing of two-thirds of the states.